Recently, from the viewpoint of environmental protection and the like, reduction in the amount of exhaust gas emissions from an engine (internal combustion engine) mounted in a vehicle and improvement of specific fuel consumption have been sought, and as vehicles that satisfy these goals, hybrid vehicles equipped with a hybrid system have been put into practical use.
Such a hybrid vehicle is provided with an engine such as a gasoline engine, and an electric motor (for example, a motor generator) that, for example, assists engine output by driving (powering) the hybrid vehicle using electric power generated by engine output or electric power stored in a battery, and in this hybrid vehicle, either or both of the engine and electric motor are used as a running power source.
Also, as a drive apparatus of the hybrid system mounted in this type of hybrid vehicle, for example, there are the drive apparatuses disclosed in the documents below.
JP 2005-264762A (referred to below as Patent Document 1)
JP 2006-273305A (referred to below as Patent Document 2)
JP H11-217025A (referred to below as Patent Document 3)
The drive apparatuses disclosed in these Patent Documents 1 to 3 are provided with a power distribution mechanism that distributes engine output to a first electric motor and an output shaft, a second electric motor that is linked to the output shaft of that power distribution mechanism, and a transmission mechanism connected thereto on the subsequent stage side thereof. The power distribution mechanism is configured with, for example, a planetary gear apparatus so as to function as a differential mechanism, and due to differential operation of the power distribution mechanism, the major part of power from the engine is mechanically transmitted towards drive wheels, and the remaining part of power from the engine is electrically transmitted using an electrical path from the first electric motor to the second electric motor. Thus, the power distribution mechanism functions as a transmission (referred to below as an electric differential unit) in which the gear ratio is electrically changed, and allows the vehicle to be run while maintaining engine operation in a region of high combustion efficiency, thereby contributing to improved specific fuel consumption.
Also, as the transmission mechanism provided on the subsequent stage side, an automatic stepped transmission mechanism is applied in the drive apparatuses of Patent Documents 1 and 2, and a belt-driven continuously variable transmission mechanism is applied in the drive apparatus of Patent Document 3.
In view of increasing the practicality of a vehicle drive apparatus in which, as described above, the electric differential unit is provided on the previous stage side, and the transmission mechanism capable of changing the gear ratio is provided on the subsequent stage side, the inventors of the present invention investigated a technique for controlling differential operation of the electric differential unit and gearshift operation of the transmission mechanism.
The inventors found that in a drive apparatus in which an automatic stepped transmission mechanism is applied as the subsequent stage side transmission mechanism, as disclosed in Patent Documents 1 and 2, because the gear ratio of this subsequent stage side transmission mechanism can only change in steps, it is not possible to realize control whereby optimal specific fuel consumption is obtained at every vehicle speed.
More specifically, engine output power is distributed by the electric differential unit, and transmitted to the drive wheels via two paths: a mechanical path where the output is transmitted towards the drive wheels as mechanical energy, and the aforementioned electrical path where the output is converted to electric energy by the electricity generating function of the first electric motor, and that electric energy is converted to mechanical energy by the second electric motor.
Here, energy loss due to the energy conversion occurs in energy transmission by the electrical path. Therefore, in order to increase motive power transmission efficiency, it is preferable to bring the electrical path amount near ‘0’, by, for example, setting the revolution speed of the first electric motor to approximately ‘0’.
However, when attempting to operate the engine along a combustion efficiency optimal line, which is a gathering of engine operation points that realizes optimal specific fuel consumption, the engine revolution speed does not change in equal proportion to the vehicle speed. Therefore, in a drive apparatus in which an automatic stepped transmission mechanism is applied as the subsequent stage side transmission mechanism, as disclosed in Patent Documents 1 and 2, the gear ratio of this subsequent stage side transmission mechanism can only change in steps, so when the engine is operated along the combustion efficiency optimal line, if an attempt is made to set the automatic stepped transmission mechanism to a predetermined gear relative to a particular vehicle speed, it may become necessary to rotate the first electric motor. That is, in response to the vehicle speed and the gear of the automatic stepped transmission mechanism, the revolution speed of the first electric motor deviates from ‘0’, and thus the electrical path amount cannot be brought near ‘0’. As a result, it is not possible to set the motive power transmission efficiency to maximum efficiency in the vehicle drive apparatus, due to generation or increase of this electric path amount.
The inventors of the present invention found that in a drive apparatus in which an automatic stepped transmission mechanism is applied as the subsequent stage side transmission mechanism, as described above, it is not possible to realize control whereby optimal specific fuel consumption is obtained at every vehicle speed. Such findings are not commonly known.
On the other hand, in Patent Document 3, although a belt-driven continuously variable transmission mechanism is applied as the subsequent stage side transmission mechanism, nothing is disclosed with respect to a control technique for the differential operation of the electric differential unit and the gearshift operation of the transmission mechanism, i.e., the technique for overall control of the gearshift operation for the vehicle drive apparatus as a whole, and Patent Document 3 does not propose a control technique that is specific to the case where a belt-driven continuously variable transmission mechanism is applied to the subsequent stage side.